1. Field of the Invention
The present invention relates to a field effect type semiconductor device, and more particularly, to a high electron mobility transistor (HEMT).
2. Brief Description of Related Prior Art
Compound semiconductor field effect transistors (FET's) using GaAs have been developed as high frequency elements, and high speed and low power consumption large scale integrated circuits (LSI's). Particularly, in HEMT's, use is made of two-dimensional electron gas formed at a heterojunction interface between an N-type AlGaAs electron supply layer and a GaAs channel layer. That is, such two-dimensional electron gas is used as carriers. In such HEMT's, if the amount of band discontinuity at the heterojunction interface is made large, the density of the two-dimensional electron gas is increased, and also, the threshold value, by which parallel conductance of the electron supply layer is started, is increased, thereby improving performance. Thus, in the prior art, in order to increase the amount of band discontinuity, the percentage composition of aluminum (Al) in the electron supply layer is increased; however, in this case, the parasitic resistance of the electron supply layer is increased particularly when the percentage composition of Al in the electron supply layer is more than 20%. Also, a deep impurity level, which is called a DX center, is created in the electron supply layer. Thus, the reliability of the HEMT's is reduced.
In order to suppress the reduction of the reliability of the HEMT's due to the DX center, there is known a pseudomorphic HEMT in which an electron supply layer is made of GaAs to increase the electron density thereof without creating a DX center, a channel layer is made of InGaAs having a high mobility, and a spacer layer between the electron supply layer and the channel layer is made of AlGaAs (see JP-A HEI1-187878)
In the above-described prior art pseudomorphic HEMT, however, low parasitic resistance, high reliability, and high electron density cannot be effectively achieved.
Also, one prior art HEMT includes an electron supply layer formed by a super lattice configuration of AlGaAs/N.sup.+ -type GaAs/AlGaAs (see JP-A-SHO60-7121).
In the above-described prior art HEMT using a super lattice configuration, the configuration is too complex, and also, the properties are too easily changed as a result of heat occurring during operation.